1. Field of Invention
Embodiments of the invention relate generally to liquid level sensor systems and methods, and more specifically, to systems and methods for reducing effects of electrical noise in liquid level sensor systems.
2. Discussion of Related Art
Withdrawing and dispensing precise volumes of a liquid without contaminating the liquid is an important part of many clinical applications and laboratory tests. A number of automated or semi-automated liquid-measuring systems are presently being used to gauge more precisely the small liquid volumes that need to be withdrawn and/or dispensed.
Typical automated systems use a motor-controlled pipette-like probe to aspirate or dispense a desired amount of fluid from or into a container. The probe is movably mounted over the container and, using a precision-controlled motor, is vertically (z-axis) lowered into the container until the tip of the probe reaches a desired level below or above the upper surface of the liquid (the meniscus). A desired amount of liquid is then withdrawn from or dispensed into the container. Such systems have been designed to: minimize/reduce cross-contamination between the contents of different containers, avoid splashing of the liquid during the aspiration (or deposition) process, and minimize/reduce the portion of the probe that must be washed.
In many instances, the automated or semi-automated liquid-measuring system does not know beforehand the level of fluid contained within the container. Nonetheless, the pipette-like probe must typically be lowered to a certain position relative to the fluid level. Several systems control the position of the probe tip without previously knowing the upper level of the liquid in the container by sensing for the upper level of the liquid in the container as the probe is being lowered. For example, a measurement can be made of some electrical phenomena associated with a change in the capacitance between the probe and the liquid in the container as the tip of the probe approaches the liquid. This measurement may identify a liquid sense event (for example, penetrating the meniscus or withdrawing from the meniscus) when the capacitance between the probe and the liquid reflects a change that is greater than a threshold reference level.
A known system and technique for capacitive-based sensing of liquid level in a z-axis controlled liquid-measuring system is described in U.S. Pat. No. 5,365,783, to Ronald A. Zweifel (the “Zweifel system”), the disclosure of which is hereby incorporated by reference. U.S. Pat. No. 3,391,547 also describes capacitive liquid level systems and methods.
In some existing systems, labware used in liquid measuring and dispensing systems can collect and store static electricity. As the probe is moved closer and closer to the container, one or more static discharge events may occur between the probe and container during probe movement. These static discharge events can cause an instantaneous change in a signal indicative of the measured capacitance which can incorrectly be detected by the system as a liquid event (for example, a false positive indication that the probe is in the liquid when in fact the probe is still positioned above the liquid). Second, laboratories employing these systems typically have fluorescent light fixtures. The electronic ballasts used by these fixtures emit high frequency electromagnetic radiation. The probes used in liquid dispensing and measuring systems can act as an antenna with respect to such radiation, resulting in electronic noise in the systems that can adversely affect measured capacitance and can cause an incorrect detection of a liquid event.